Incident Overview
On an unspecified date, a float-equipped de Havilland DHC-6 Twin Otter (serial number 2) departed Langara at approximately 1350 Pacific daylight time. The aircraft was carrying 8 passengers and 2 pilots on a 35-minute visual flight rules (VFR) flight to Alliford Bay in the Queen Charlotte Islands, British Columbia. About 20 minutes after departure, the forward fuel tank low-level caution light illuminated, even though the forward fuel gauge indicated 310 pounds of fuel. The captain consulted the emergency checklist, which indicated that this situation could be caused by a blocked ejector pump slowing fuel transfer. He continued the flight, assessing that gravity feed would suffice. About five minutes later, low fuel pressure caution lights came on and the number 2 engine stopped. The forward fuel gauge indicated about 200 pounds. The captain decided to land on Yakoun Lake. The aircraft landed without further event, but the number 1 engine also stopped as the captain attempted to taxi. The fuel tanks were found empty.
Pre-Flight Fuel Calculations
On the day of the incident, the operation involved flying passengers between Alliford Bay and Langara. Pre-flight fuel calculations by the pilots indicated each flight would consume 340 pounds of fuel: 50 pounds for start, taxi, and climb, plus 290 pounds for en route, based on 29 minutes at 600 pounds per hour. Prior to the first flight, the captain used a dipstick to find 550 pounds of fuel in the tanks; the fuel gauge matched the dip, but the aft tank gauge was intermittent. The captain added 600 pounds and departed for Langara. After returning, he added another 600 pounds. No refueling records were kept. Actual flight times were 39, 39, and 43 minutes respectively. Fuel flow readings indicated 600 pounds per hour burn.
Fuel System Issues
The journey logbook entries for the first flight showed a fuel weight of 850 pounds, but the captain reported 1,150 pounds, making takeoff weight 95 pounds above maximum certificated gross weight. On the incident flight, the logbook indicated 860 pounds. The forward fuel gauge was believed accurate, but the aft gauge was inoperative. The captain assumed equal fuel in both tanks. Post-incident calibration revealed the forward gauge read about 14% less than actual fuel above 100 pounds and 16-20% low at lower quantities. The low-level caution light functioned normally. The aft tank low-level caution light did not illuminate during the flight.
Aftermath
After landing, another Twin Otter delivered an aircraft maintenance engineer (AME) and two drums of fuel. The aircraft was refueled and engines restarted, then ferried to Alliford Bay. The AME later rectified the fuel gauge problems by replacing electrical cable ends and connectors. Investigators conducted calibration testing on July 8, 1997. The pilot was certified and qualified; the first officer was new to the company. The minimum equipment list (MEL) for dispatch with one inoperative gauge required specific procedures, which were not followed. The company's base AME noted after the incident that both dip and gauge were in error compared to known fuel loaded.